Accuracy of Stationary Bike Calorie Counters

I hate using a trainer and I hate using a stationary bike even more, but due to some nasty snowfall and other complications, I've been forced wrack my knees on the stationary bikes indoors. Anyway, I average 18 mph for an hour on medium to high resistance around 180-220 Watts, but at the end, this thing tells me I've burned 1,200-1,400 calories. This can't be right, I would guess around half of that maybe.

My simple physics tells me: energy = power x time = (200 J/s)*(1 hr)*(3600 s/hr)*(0.000239 kcal/J)
= 172 kcal (of mechanical work required)
And factoring in my body's efficiency, the amount of mechanical work produced per energy used (because much heat is produced, etc.) roughly equal to 0.25, (172 kcal)/(0.25) = 688 kcal,
which is almost exactly half of what the display tells me.

So I would think not to trust these numbers, but does anyone here know if there is a way to find out how the machines come up with these figures? Or do you know of a more reliable way to track the amount of energy exhausted during a ride? I don't really care how many calories I burn. My simple training program of ride hard and if you're hungry, eat; thirsty, drink, etc. has worked since high school. I just get the idea that a lot people are probably getting of these bikes thinking they just burned 1000 calories, go eat a ton, and actually end up putting on a bunch of fat because of too much reliance on these calorie counters.

I hate using a trainer and I hate using a stationary bike even more,... Anyway, I average 18 mph for an hour on medium to high resistance around 180-220 Watts, but at the end, this thing tells me I've burned 1,200-1,400 calories. This can't be right, I would guess around half of that maybe.

My simple physics tells me: energy = power x time = (200 J/s)*(1 hr)*(3600 s/hr)*(0.000239 kcal/J)
= 172 kcal (of mechanical work required)
And factoring in my body's efficiency, the amount of mechanical work produced per energy used (because much heat is produced, etc.) roughly equal to 0.25, (172 kcal)/(0.25) = 688 kcal,
which is almost exactly half of what the display tells me.

to me, your calcs seem right there for about a 160ish lb rider.
I also hate the 'goin nowhere' of stationary bikes. Interesting thing is that the bikes in my gym come pretty close to 'your' numbers when I ride them at about the same perceived effort as when ridin outside.
Still its all just some general measure to judge against at another day/session. Really getting close to what might be accurate numbers is only gonna happen with a power meter.
The numbers you see might be 'enhanced' on the bikes in your gym due to real 'marketing'. A higher number gives the sweat-ee a bit more ecnouragement to keep it up. I remember overhearing a couple of stationary riders in my club talkin and takin exception to the fact that they were on the bikes for almost an hour and had only burned 320 calories ... They thought something was wrong with the machines, cause consuming something with 320 calories is no big deal...
I mean, I've watched people hang from those bikes for what seemed my entire gym session, and never break a sweat. Not sure how you can do that.